Such metal sheets are more particularly intended to manufacture parts for the automobile industry, but are not limited thereto.
The metal coatings, essentially comprising zinc and aluminum in small proportions (typically approximately 0.1 wt %), are traditionally used for good corrosion protection. These metal coatings are currently subject to competition in particular from coatings comprising zinc, magnesium and aluminum.
Such metal coatings will be globally referred to hereinafter as zinc-aluminum-magnesium or ZnAlMg coatings.
Adding magnesium significantly increases the resistance of these coatings to corrosion, which may make it possible to reduce their thickness or increase the corrosion protection guarantee over time.
The coils of metal sheets with such surface coatings may reside in storage hangars for several months, and that surface must not be altered by the appearance of surface corrosion, before being shaped by the end user. In particular, no beginning of corrosion must appear, regardless of the storage environment, even in case of exposure to the sun and/or a wet or even salty environment.
Standard galvanized products, i.e., the coatings of which essentially comprise small proportions of zinc and aluminum, are also subjected to these stresses and are coated with a protective oil that is generally sufficient to provide protection against corrosion during storage.